Redaction: In spite of the vacant years (v. 7), Mormon acknowledges that these years were not event-free (v. 8). Rather, he is making narrative selections that fit his overall purpose. The events of these years did not interest Mormon. The most important part of his story is approaching.
Text: Although Nephi3 is among the period’s religious rulers, there is no indication that he is a political ruler. He creates a separate record which is then “engraven on the plates which were called the plates of Nephi” (3 Ne. 5:10); he does not record directly on those large plates (though we may safely hypothesize that it was added to that larger record at a later date).
Mormon has access to Nephi3’s record because it is included in the larger-plate tradition. He elects to follow Nephi’s account rather than the more official political version. I hypothesize that the many “great and marvelous” things were probably secular; Nephi’s record would have held more interest for Mormon.